New structures and fabrication processes promise to boost performance and reduce the cost of optoelectronic ICs, which convert electrical signals representing voice, data, and video to and from light. The greatest advantage of optoelectronic chips lies in their data-transfer rates--potentially up to about 5 gigabits per second, far higher than what discrete optical and electronic components allow. Such high speeds can be obtained with low noise, amounting to about 1 error in 1 billion to 10 billion bits. In addition, a single optoelectronic IC can take on many functions--for example, light-emitting and light-receiving devices handling several different wavelengths for multichannel communications, along with related signal-processing functions.
Monolithic integration of semiconductor amplifiers together with optical multiplexers can be used to build light sources for wavelength division multiplexed (WDM) systems that are capable of producing a comb of exactly spaced frequencies. For economical application of monolithic integration to such an application, however, it is necessary to dielectrically isolate the adjacent optical amplifiers. In a paper entitled "Digitally Tunable Channel Dropping Filter/Equalizer Based on Waveguide Grating Router and Optical Amplifier Integration" (IEEE Photonics Technology Letters, Vol. 6, No. 4, April 1994), M. Zirngibl, C. H. Joyner, and B. Glance evaluated a tunable filter consisting of an array of waveguide grating multiplexers monolithically integrated with semiconductor optical amplifiers on an n+INP substrate. The optical amplifiers are integrated on both sides of the wavelength grating router and are spaced approximately 125 .mu.m apart on each side. A transverse cross sectional view of the tunable filter 1, showing the arrangement of adjacent optical amplifiers, is shown in FIG. 1.
Each optical amplifier comprises a doped section of waveguide with controllable optical transmissivity. The doping is such that an appropriately configured semiconductor junction is defined in each optical amplifier. These sections are optically active in that application of electrical energy to those sections will cause them to become transmissive to the flow of optical energy and will even provide some degree of gain to optical signals flowing through them. When electrical bias current above a predetermined threshold is applied, lasing action begins. These doped sections of waveguide are substantially opaque to the transmission of light when there is no applied electrical stimulation.
As shown in FIG. 1, laser cavity active regions are formed of quantum wells (QW), 2, 3 and bandgap barriers 4, 5 lattice matched to InP. Semi-insulating regions 6 are provided between the amplifiers 7. A heavily doped p+ cap layer 8 serves as the contact layer and has metallized contacts 9 formed thereon.
In the aforementioned paper, it was reported that electrical isolation between adjacent optical amplifiers was only 50 .OMEGA.. Significant crosstalk and power penalties, as evidenced by sidelobes in a graphical representation of output power versus input wavelength, can be attributed to weak electrical isolation. Ion implantation of impurities into the cap layer might be employed to create hole-filled isolating regions. However, such a technique is only expected to yield an inter-device resistance on the order of several k.OMEGA.. Of course, implantation requires additional mask layers and other processing steps, which steps may be destructive to other components. It is also anticipated that some of the impurities may diffuse into the underlying InP layer during implantation, thereby causing the InP layer to act as an attenuator.